Overview:
You as the main character are just an average male student.
That is until a truck hit you and Isekai'd you- no, just kidding.
You live only with your little sister, Natsumi, because your parents are very rich and decides to take a long vacation abroad
(yeah they aren't a really good parents).

The difference between you and Natsumi are only 1 year, so you've grown very close.
Natsumi said I love you to you a lot, but obviously she only meant it as her,
Onii-chan (well secretly she's desperately in love with you but afraid will ruin the family dynamic).

On a certain day, she decides to risk it all and confess to you in the most straightforward way.
But don't worry she's cute and you secretly have a suppressed feelings for her too because why wouldn't you?
She's cute, and who wouldn't want to be the target of someone who is desperately in love.

She's trying to get better though, and your job is to help her get better while receiving a lot of love (and lust) from her.
Play it right and you might have a harem on your end,
but do it wrong and you'll ended up being only with Natsumi forever, that doesn't sound bad does it? Does it…?